Researchers at the National Institute of Standards and Technology (NIST) have
cultivated many thousands of nanocrystals in what looks like a pinscreen or "pin
art" on silicon, a step toward reliable mass production of semiconductor
nanowires for millionths-of-a-meter-scale devices such as sensors and
lasers.

Colorized micrograph of semiconductor nanowires grown at NIST in a precisely
controlled array of sizes and locations.

NIST researchers grow nanowires made of semiconductors—gallium nitride
alloys—by depositing atoms layer-by-layer on a silicon crystal under high
vacuum. NIST has the unusual capability to produce these nanowires without using
metal catalysts, thereby enhancing luminescence and reducing defects. NIST
nanowires also have excellent mechanical quality factors.

The latest experiments, described in Advanced Functional Materials,*
maintained the purity and defect-free crystal structure of NIST nanowires while
controlling diameter and placement better than has been reported by other groups
for catalyst-based nanowires. Precise control of diameter and placement is
essential before nanowires can be widely used.

The key trick in the NIST technique is to grow the wires through precisely
defined holes in a stencil-like mask covering the silicon wafer. The NIST
nanowires were grown through openings in patterned silicon nitride masks. About
30,000 nanowires were grown per 76-millimeter-wide wafer. The technique
controlled nanowire location almost perfectly. Wires grew uniformly through most
openings and were absent on most of the mask surface.

Mask openings ranged from 300 to 1000 nanometers (nm) wide, in increments of
100 nm. In each opening of 300 nm or 400 nm, a single nanowire grew, with a
well-formed hexagonal shape and a symmetrical tip with six facets. Larger
openings produced more variable results. Openings of 400 nm to 900 nm yielded
single-crystal nanowires with multifaceted tops. Structures grown in 1,000-nm
openings appeared to be multiple wires stuck together. All nanowires grew to
about 1,000 nm tall over three days.

NIST researchers analyzed micrographs to verify the uniformity of nanowire
shape and size statistically. The analysis revealed nearly uniform areas of
wires of the same diameter as well as nearly perfect hexagonal shapes.

Growing nanowires on silicon is one approach NIST researchers are exploring
for making "nanowires on a chip" devices. Although the growth temperatures are
too high—over 800 degrees Celsius—for silicon circuitry to tolerate, there may
be ways to grow the nanowires first and then protect them during circuitry
fabrication, lead author Kris Bertness says. The research was partially
supported by the Defense Advanced Research Projects Agency (DARPA) Center on
NanoscaleScience and Technology for Integrated Micro/Nano-Electromechanical
Transducers (iMINT) at the University
of Colorado at Boulder.